There is a marine product called Coosa Board - similar - never rot. NOT cheap. Depending on year and model most floors were 5/8 or 3/4 plywood (or Dog help us - OSB
If you choose a high quality plywood, get whatever fits in the C-channel, but get high quality stuff. Many recommend against marine plywood because it's got chemicals that could react badly with the aluminum C-channel. A lot of people use various water-proofing treatments - fiberglass resin thinned down is popular, as are various epoxy sealers. Use the Google option under the SEARCH function to look up what others have done. Sealing the edges is very important for those of us who live in wet or humid climates. Allowing for moisture that does get between the walls or in the belly pan to drain out is crucial too. HUMIDITY can do it just as well as outright leaks.
I don't believe I'd go with thicker stuff than was original, but some who have done so use a router to ease the top edge of the plywood down 1/16 of an inch or even 1/8th.
Do some research and thinking about it - you're going to have to live with the results for the next 50 years!
Good luck - good hunting.